Thomas Haller authored May 01, 2019 and Lubomir Rintel committed May 21, 2019

I think initializing structs should (almost) be always done with designated
initializers, because otherwise it's easy to get the order wrong. The
problem is that otherwise the order of fields gets additional meaning
not only for the memory layout, but also for the code that initialize
the structs.

Add a macro NM_VARIANT_ATTRIBUTE_SPEC_DEFINE() that replaces the other
(duplicate) macros. This macro also gets it right to mark the struct as
const.

(cherry picked from commit 86dc50d4)